1. Field of the Invention
The present invention relates to a tension measuring device and to a machine having such a device for measuring tension or tension changes in a paper web. The device includes a number of measurement bodies and a number of fluid supply devices intended to apply a fluid pressure to the web. The fluid supply devices are so arranged as to subject the web to an essentially constant fluid pressure in order to provide a fluid cushion between the web and the measurement bodies. The measurement bodies are so arranged as to be capable of displacement, and are so arranged as to apply pressure to the web with the help of application devices and without making contact with the web. The position of the measurement bodies in a direction normal to the web is altered depending on variations in the constitution of the web.
The present invention also relates to a method for measuring tension or a tension change in a material web utilizing at least one measuring device having a number of measurement bodies and a number of fluid supply devices, which are intended to apply a fluid pressure to the web.
2. Description of the Related Art
A major problem in today's paper manufacture is to ensure that the finished paper maintains a high and even quality. A number of factors can cause parts of a roll of paper to exhibit a quality that deviates from the anticipated quality. For example, an uneven distribution of moisture can cause the paper not to exhibit the flat, even, and uniform surface that is desired. In conjunction with manufacture, it is very important for the paper web to possess the correct web tension profile. The paper must accordingly exhibit a web tension profile that is adapted so that the paper has a high and even quality such that the paper web is not deformed. A series of different sub-processes in the forming and drying of the paper web can create problems in the form of slack or taut paths, creasing, and even folding if the creases are so large that they do not disappear in the process. Throughout the process, and up to the point at which the paper is rolled up, an effort is made to maintain a certain web tension, which stretches the material. Creasing can occur, however, if the variations between different segments of the width of the web are large. When the final user then unrolls the paper from the roll, the web tension is often released, and the slack paths, that is to say segments with a small web tension relative to the surrounding segments, manifest themselves as negative web tension, which is associated with changes in form and operability problems.
Various solutions to the problem of measuring the actual web tension in webs are presented below:
German published application DE 101 48 309 A1 shows an arrangement in which air flows out through a nozzle and over a web, whereby the deflection in the web caused by the airflow is registered with the help of, for example, laser triangulation or ultrasound sensors.
European published application EP 1 321 754 A1 illustrates a method to measure the tension across the entire width of a web with the help of traversing measurement elements. The measurement element is bow-shaped, which means that the air that is carried along by the web is forced between the web and the measurement element and produces an air cushion. The pressure in the air cushion is measured with the help of orifices positioned on the measurement rail.
Each of the above-mentioned documents relates to different devices which measure the tension in paper webs. Those devices nevertheless require the positioning of sensors in close association with the web in order either to measure the vertical deflection of the web due to the fluid pressure of the nozzles, or to measure the fluid pressure between the web and the nozzles.
U.S. Pat. Nos. 3,496,744; 4,031,741; and 3,850,024 show measurement devices on rolling mills for measuring the tension distribution in moving metal or paper webs by continuously measuring unevennesses in the web and controlling the rolling mill accordingly. Tension measurement is performed by causing the nozzles to subject a web to an approximately constant air pressure. If a web exhibits unevenness, the vertical position of the nozzles is changed accordingly, which is detected by various types of sensors.
The devices described in the above-mentioned documents may possibly bring about good measurement of webs of metal, although they do not give a particularly reliable measurement of webs of paper or other more flexible materials.
An object of the present invention is thus to provide a measuring device which measures the tension of a moving or stationary paper web in a simple, reliable fashion and by simple means.